This invention relates to a circuit configuration for actuating a safety relay such as the main relay or power supply relay of an electronically controlled brake system of an automotive vehicle. At least two electronically operable switches, particularly transistors connected in series are provided with a monitoring circuit which interrupts the power supply of the safety relay by actuating the switches if there is a defect.
An anti-lock-controlled brake system of this type is disclosed in German Patent (DE-PS) No. 26 12 356 (corresponding with U.S. Pat. No. 4,133,321) and in German Published Patent Application (DE-OS) No. 32 34 637 (corresponding to U.S. Pat. No. 4,546,437). The respective brake systems are arranged to ensure after the safety relay has switched off or has dropped out, all braking pressure control valves adopt or maintain their rest positions in which uncontrolled braking operations are possible without restrictions. Thus, defects in the electronic system cannot put the brake out of operation. It therefore must be ensured that it is possible to switch off the main relay which is important for the operation of the brake and, therefore, for the safety of the vehicle. Therefore, at least two transistors connected in series are inserted into the power supply path of the safety relay or main relay so as to maintain the safety function by means of the second transistor even in case of failure or of a blow-out of the other transistor.
By means of the two or more transistors connected in series it is achieved that it will be possible to switch off the safety relay even if one of the two switches will remain conductive permanently, such a defect, however, is not detectable although there does not exist any longer the safety stand-by function intended by the redundant arrangement of the switches.
It is thus an object of the present invention to eliminate this disadvantage and to provide a circuit configuration for actuating a safety relay which also will allow the failure of a redundant switch to be detected.